The objectives of this study are to characterize possible age-related differences in the in vitro responses of murine lymphohemopoietic cells to mitogen and their differential susceptibility to freezing damage. A prototype controlled rate cooling system is currently being tested on lymphocytes of young mice. A higher than expected recovery of functional cells was observed at several cooling rates, which may be related to the cooling and freezing program employed. The system relies on the cell suspension temperature that is sensed to drive the injection of liquid N2 into the cooling chamber. This method is not available in present freezing systems. A circannual rhythm of lymphocyte responsiveness to mitogens in vitro was observed in a third year. Superimposed upon this rhythm were seasonal changes in the dose-response curve of murine lymphocytes to Concanavalin A. BIBLIOGRAPHIC REFERENCE: Brock, M.A.: Free-running rhythmicity in the life spans of hydranths of the marine cnidarian, Campanularia flexuosa. J. Interdisciplinary Cycle Res. 7: 269-278, 1976.